A cancer is a clonal proliferation of cells produced as a consequence of cumulative genetic damage that finally results in unrestrained cell growth, tissue invasion and metastasis (cell transformation). Regardless of the type of cancer, transformed cells carry damaged DNA in many forms: as gross chromosomal translocations or, more subtly, as DNA amplification, rearrangement or even point mutations.
Some oncogenic mutations may be inherited in the germline, thus predisposing the mutation carrier to an increased risk of cancer. However, in a majority of cases, cancer does not occur as a simple monogenic disease with clear Mendelian inheritance. There is only a two- or threefold increased risk of cancer among first-degree relatives for many cancers (Mulvihill J J, Miller R W & Fraumeni J F, 1977, Genetics of human cancer Vol 3, New York Raven Press). Alternatively, DNA damage may be acquired somatically, probably induced by exposure to environmental carcinogens. Somatic mutations are generally responsible for the vast majority of cancer cases.
Studies of the age dependence of cancer have suggested that several successive mutations are needed to convert a normal cell into an invasive carcinoma. Since human mutation rates are typically 10.sup.-6 /gene/cell, the chance of a single cell undergoing many independent mutations is very low (Loeb L A, Cancer Res 1991, 51: 3075-3079). Cancer nevertheless happens because of a combination of two mechanisms. Some mutations enhance cell proliferation, increasing the target population of cells for the next mutation. Other mutations affect the stability of the entire genome, increasing the overall mutation rate, as in the case of mismatch repair proteins (reviewed in Arnheim N & Shibata D, Curr. Op. Genetics & Development, 1997, 7: 364-370).
An intricate process known as the cell cycle drives normal proliferation of cells in an organism. Regulation of the extent of cell cycle activity and the orderly execution of sequential steps within the cycle ensure the normal development and homeostasis of the organism. Conversely, many of the properties of cancer cells--uncontrolled proliferation, increased mutation rate, abnormal translocations and gene amplifications--can be attributed directly to perturbations of the normal regulation or progression of the cycle. In fact, many of the genes that have been identified over the past several decades as being involved in cancer, can now be appreciated in terms of their direct or indirect role in either regulating entry into the cell cycle or coordinating events within the cell cycle.
Recent studies have identified three groups of genes which are frequently mutated in cancer. The first group of genes, called oncogenes, are genes whose products activate cell proliferation. The normal non-mutant versions are called protooncogenes. The mutated forms are excessively or inappropriately active in promoting cell proliferation, and act in the cell in a dominant way in that a single mutant allele is enough to affect the cell phenotype. Activated oncogenes are rarely transmitted as germline mutations since they may probably be lethal when expressed in all the cells. Therefore oncogenes can only be investigated in tumor tissues.
Oncogenes and protooncogenes can be classified into several different categories according to their function. This classification includes genes that code for proteins involved in signal transduction such as: growth factors (i.e., sis, int-2); receptor and non-receptor protein-tyrosine kinases (i.e., erbB, src, bcr-abl, met, trk); membrane-associated G proteins (i.e., ras); cytoplasmic protein kinases (i.e., mitogen-activated protein kinase--MAPK-family, raf, mos, pak), or nuclear transcription factors (i.e., myc, myb, fos, jun, rel) (for review see Hunter T, 1991 Cell 64: 249; Fanger G R et al., 1997 Curr.Op.Genet.Dev.7: 67-74; Weiss F U et al., ibid. 80-86).
The second group of genes which are frequently mutated in cancer, called tumor suppressor genes, are genes whose products inhibit cell growth. Mutant versions in cancer cells have lost their normal function, and act in the cell in a recessive way in that both copies of the gene must be inactivated in order to change the cell phenotype. Most importantly, the tumor phenotype can be rescued by the wild type allele, as shown by cell fusion experiments first described by Harris and colleagues (Harris H et al.,1969, Nature 223: 363-368). Germline mutations of tumor suppressor genes may be transmitted and thus studied in both constitutional and tumor DNA from familial or sporadic cases. The current family of tumor suppressors includes DNA-binding transcription factors (i.e., p53, WT1), transcription regulators (i.e., RB, APC, probably BRCA1), protein kinase inhibitors (i.e., p16), among others (for review, see Haber D & Harlow E, 1997, Nature Genet. 16: 320-322).
The third group of genes which are frequently mutated in cancer, called mutator genes, are responsible for maintaining genome integrity and/or low mutation rates. Loss of function of both alleles increase cell mutation rates, and as consequence, protooncogenes and tumor suppressor genes may be mutated. Mutator genes can also be classified as tumor suppressor genes, except for the fact that tumorigenesis caused by this class of genes cannot be suppressed simply by restoration of a wild-type allele, as described above. Genes whose inactivation may lead to a mutator phenotype include mismatch repair genes (i.e., MLH1, MSH2), DNA helicases (i.e., BLM, WRN) or other genes involved in DNA repair and genomic stability (i.e., p53, possibly BRCA1 and BRCA2) (For review see Haber D & Harlow E, 1997, Nature Genet. 16: 320-322; Fishel R & Wilson T. 1997, Curr.Op.Genet.Dev.7: 105-113; Ellis NA,1997 ibid.354-363).
The recent development of sophisticated techniques for genetic mapping has resulted in an ever expanding list of genes associated with particular types of human cancers. The human haploid genome contains an estimated 80,000 to 100,000 genes scattered on a 3.times.10.sup.9 base-long double-stranded DNA. Each human being is diploid, i.e., possesses two haploid genomes, one from paternal origin, the other from maternal origin. The sequence of a given genetic locus may vary between individuals in a population or between the two copies of the locus on the chromosomes of a single individual. Genetic mapping techniques often exploit these differences, which are called polymorphisms, to map the location of genes associated with human phenotypes.
One mapping technique, called the loss of heterozygosity (LOH) technique, is often employed to detect genes in which a loss of function results in a cancer, such as the tumor suppressor genes described above. Tumor suppressor genes often produce cancer via a two hit mechanism in which a first mutation, such as a point mutation (or a small deletion or insertion) inactivates one allele of the tumor suppressor gene. Often, this first mutation is inherited from generation to generation.
A second mutation, often a spontaneous somatic mutation such as a deletion which deletes all or part of the chromosome carrying the other copy of the tumor suppressor gene, results in a cell in which both copies of the tumor suppressor gene are inactive.
As a consequence of the deletion in the tumor suppressor gene, one allele is lost for any genetic marker located close to the tumor suppressor gene. Thus, if the patient is heterozygous for a marker, the tumor tissue loses heterozygosity, becoming homozygous or hemizygous. This loss of heterozygosity generally provides strong evidence for the existence of a tumor suppressor gene in the lost region.
By genotyping pairs of blood and tumor samples from affected individuals with a set of highly polymorphic genetic markers, such as microsatellites, covering the whole genome, one can discover candidate locations for tumor suppressor genes. Due to the presence of contaminant non-tumor tissue in most pathological tumor samples, a decreased relative intensity rather than total loss of heterozygosity of informative microsatellites is observed in the tumor samples. Therefore, classic LOH analysis generally requires quantitative PCR analysis, often limiting the power of detection of this technique. Another limitation of LOH studies resides on the fact that they only allow the definition of rather large candidate regions, typically spanning over several megabases. Refinement of such candidate regions requires the definition of the minimally overlapping portion of LOH regions identified in tumor tissues from several hundreds of affected patients.
Another approach to genetic mapping, called linkage analysis, is based upon establishing a correlation between the transmission of genetic markers and that of a specific trait throughout generations within a family. In this approach, all members of a series of affected families are genotyped with a few hundred markers, typically microsatellite markers, which are distributed at an average density of one every 10 Mb. By comparing genotypes in all family members, one can attribute sets of alleles to parental haploid genomes (haplotyping or phase determination). The origin of recombined fragments is then determined in the offspring of all families. Those that co-segregate with the trait are tracked. After pooling data from all families, statistical methods are used to determine the likelihood that the marker and the trait are segregating independently in all families. As a result of the statistical analysis, one or several regions are selected as candidates, based on their high probability to carry a trait causing allele. The result of linkage analysis is considered as significant when the chance of independent segregation is lower than 1 in 1000 (expressed as a LOD score&gt;3). Identification of recombinant individuals using additional markers allows further delineation of the candidate linked region, which most usually ranges from 2 to 20 Mb.
Linkage analysis studies have generally relied on the use of microsatellite markers (also called simple tandem repeat polymorphisms, or simple sequence length polymorphisms). These include small arrays of tandem repeats of simple sequences (di-tri-tetra-nucleotide repeats), which exhibit a high degree of length polymorphism, and thus a high level of informativeness. To date, only just more than 5,000 microsatellites have been ordered along the human genome (Dib et al., Nature 1996, 380: 152), thus limiting the maximum attainable resolution of linkage analysis to ca. 600 kb on average.
Linkage analysis has been successfully applied to map simple genetic traits that show clear Mendelian inheritance patterns. About 100 pathological trait-causing genes were discovered by linkage analysis over the last 10 years.
However, linkage analysis approaches have proven difficult for complex genetic traits, those probably due to the combined action of multiple genes and/or environmental factors. In such cases, too large an effort and cost are needed to recruit the adequate number of affected families required for applying linkage analysis to these situations, as recently discussed by Risch, N. and Merikangas, K. (Science 1996, 273: 1516-1517). Finally, linkage analysis cannot be applied to the study of traits for which no available large informative families are available. Typically, this will be the case in any attempt to identify trait-causing alleles involved in sporadic cases.
Despite the significant progress in identifying genes associated with cancer, there remains a need for the identification and characterization of additional cancer genes. The present invention relates to the identification of a gene associated with prostate cancer, identified as the PG1 gene, and reagents, diagnostics, and therapies related thereto.